a) Field of the Invention
The invention is directed to an arrangement for severing brittle materials by means of a laser by inducing thermomechanical stresses as is known generically from WO96/20062.
b) Description of the Related Art
Laser methods in which the material is split by inducing thermal stresses rather than removing material rely on the principle of limited local heating below the softening temperature of the material so that compressive stresses occur in the material and subsequent sudden cooling by a directed coolant jet so that tensile stresses are generated. The forces occurring in the material lead to a cleavage break.
Apart from a laser method of the type mentioned above, WO96/20062 also discloses an arrangement for implementing such a method. The arrangement comprises an irradiation portion having a laser and a focusing system which is arranged along the optical axis of the laser, a mechanism which is displaceable relative to the irradiation portion and by which a coolant is brought into the cutting zone, meant for fixating the material to be cut, means for the displacement of the laser and the coolant relative to the workpiece, and a control device.
Many other publications are directed to methods by which the beam density distribution is the beam spot or the beam spot shape are optimized in order to improve the quality and speed of the severing process. In a corresponding manner, the arrangement used for implementing the method are distinguished substantially only by different beam-shaping elements and beam-guiding elements. Such arrangements basically have the following features in common.                A laser radiation source, basically with controllable radiation output and radiation duration. Depending on the laser, the emitting beam bundle can have, in particular, different beam cross-sectional shapes and different beam density distribution in addition to a different wavelength and radiation output.        A laser head in which optical means are provided for focusing the laser beam bundle in direction of the workpiece surface. In certain cases, these optical means can also be used for beam shaping and beam guidance or to change a beam density distribution.        Means for the relative movement of the laser head and workpiece along a severing line.        A coolant device with a coolant nozzle that is arranged behind the laser nozzle in the movement direction at an adjustable or fixed distance.        A control device by which, among other things, the laser head and the coolant nozzle connected to the latter are positioned in direction of the severing line.        
It is a priority in the designing of arrangements of this kind that these arrangements must operate precisely and at a high process speed in permanent operation and under industrial conditions.
Downtimes must be kept as short as possible in order to increase the efficiency of the method at given process speed, i.e., at a given time period for carrying out a severing cut. Downtimes are nonoperating periods for maintenance, setup, and positioning of the tool and workpiece in a starting position.
If only one severing cut is to be carried out on a workpiece, the downtime required for exchanging the severed workpiece for a new workpiece can also be used to bring the tool into the starting position.
Downtimes a particularly long when a plurality of starting positions must be set up in order to cut a workpiece into a plurality of individual parts, e.g., when covering a wafer disk into a plurality of chips. In this connection, the wafer disk is cut in x-direction into a plurality of individual strips of equal width which are subsequently cut in y-direction into a plurality of chips of equal length.
After every severing cut, the tool, i.e., the laser head with the coolant nozzle, and the workpiece must together be brought into a new starting position. For this purpose, either the laser head which is fixedly connected to the coolant nozzle over the stationary workpiece can be brought back to the starting edge of the preceding severing cut and offset by the strip width (forward feed movement) and the wafer disk is positioned under the stationary laser head in a corresponding manner, or the severing cut is started at the ending edge of the preceding cut, which then only requires relative offsetting by the strip width but necessitates a repositioning of the cooling nozzle relative to the laser head so that the cooling nozzle is arranged behind the laser head in the cutting direction.
When the severing cuts are carried out proceeding from the same starting edge, the downtime is high, substantially comprising the time required for positioning the laser head at the beginning of a severing cut.
When the severing cuts are carried out from alternate sides, a high downtime results substantially from the time required to reposition the coolant nozzle relation to the laser head so that it is arranged behind the laser head in each instance.